It is very common for battery packs which are used in portable communication devices, such as two-way radios, to have a thermistor and a battery capacity resistor. The thermistor is used by a battery charger during the charging of the battery, to determine if the battery is being charged properly. While the capacity resistor is used by the charger to determine the capacity of the battery, prior to the battery being charged. The battery charger upon determining the battery capacity (e.g., 1000 milli-amp-hour maH, etc.) will select the proper charging rate to use, in order to optimally charge the battery pack.
Referring to FIG. 1, there is shown a prior art battery charging scheme consisting of a charger 102, radio battery pack 106 and radio 104. Radio 104 contains positive (B+) and negative (B-) battery terminals which are coupled to radio battery 106 via battery contacts 116 and 114, respectively. Battery 106 contains one or more battery cells 108, which determine the voltage and current capacity of battery 106. Also included as part of the battery 106, are protection diode 118, a battery temperature indicator such as thermistor (Rt) 112 and a battery capacity indicator, such as resistor (Rc) 110.
Charger 102 consists of a charger monitor circuit 128, which can consist of a well known microprocessor or microcontroller as known in the art and appropriate control software. Charger monitor circuit 128 controls charger control circuit 130 which provides current to battery 106 in order to charge the battery. A control signal is transmitted by charger monitor circuit 128 to charger control circuit 130 via bus 140, the control signal informs charger control circuit 130 on how much current to source via line 129 to battery 106.
Charger monitor circuit 128 contains three analog to digital (A/D) ports 120, 122 and 124. A/D port 120 monitors the voltage on the B+ line. A/D port 122 senses the resistance of capacity resistor Rc 110 and A/D port 124 in turn senses the resistance of thermistor Rt 112, as its resistance changes once the battery begins charging. A/D ports 122 and 124 include external pull-up resistors which are used to determine the resistance of Rc 110 and Rt 112, by determining the voltage level at A/D ports 122 and 124, respectively.
Charger 102 and battery 106 in the prior art scheme are required to have 4 lines connecting the charger 102 and battery 106. These lines being B+ line 132 which provides the charging current to the battery pack, Rc line 134 which is used to sense the capacity resistor 110, thermistor sense line 1 36 used to sense the resistance value of thermistor 112, and B- (ground) line 138.
The problem with the prior art battery charging and sensing scheme is that the charger requires 4 contacts and the battery requires 4 contacts in order for the battery and charger to mate with each other (lines 132-138) and perform all the required charging functions. Battery charger contacts tend to be expensive, given that they are usually finger contacts which are typically gold plated in order to provide good electrical connection to battery pack 106. The battery contacts also tend to be gold plated. A need thus exists for a method and apparatus that can accomplish the battery characterization functions (e.g., battery temperature and capacity determinations) performed by the prior art battery scheme, while reducing the number of battery contacts that are utilized to accomplish the required functions.